Wireless Communications: The Future

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Wireless CommunicationsThe Future

• Professor Song Chong
• Network Systems Laboratory
• EECS, KAIST
• song_at_ee.kaist.ac.kr

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Current Position

• A wide range of wireless devices
• Mobile
• Fixed
• Short-range
• Broadcasting
• Mobile
• Cellular
• 2G
• 3G
• WiMax
• So-called 4G
• Mobile Mesh
• Emerging technologies cognitive radio (CR),
  software-defined radio (SDR)

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Current Position

• Fixed
• Point-to-point
• Point-to-multipoint
• Fixed mesh
• Short-range
• W-LANs
• 802.11 family
• Zigbee
• W-PANs
• BlueTooth
• High-speed variants such as WiMedia/UWB
• RFIDs
• Broadcasting
• Analog and digital broadcasting
• Mobile broadcasting

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3G Cellular

• In 2006, 3G systems were starting to be widely
  deployed.
• W-CDMA (European standard), Cdma2000 (US
  standard), TD-SCDMA (Chinese standard)
• 3G will eventually take over from 2G but the
  growth may occur more because the operators
  push the new technology than the subscribers
  demand it.
• Realistic data rates of 3G will not go beyond
  around 400 Kbps.
• The lifetime of 3G will be around 20 years.
• The wide use of new services 3G offers such as
  video call and streaming will take 10 years.
• As of today, the benefits of 3G were being
  realized as increased voice call capacity.
• 3G will face competition from other technologies.
• W-LAN for hotspot and indoor voice and data
• WiMax for outdoor voice and data, although this
  is not certain

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4G Cellular

• The deployment of 4G sometime around 2014-2018
  might look like a fairly certain bet.
• Definition of 4G is still opaque, 4G is likely to
  be different from 3G (not just be a new air
  interface), and perhaps may not even emerge.

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4G Cellular

• Each generation has accepted a smaller cell size
  in return for a higher data rate.
• Higher data rate -gt more spectrum -gt higher
  frequency -gt lower propagation range -gt smaller
  cell
• The next step in the process, where 4G might
  logically fit, is already taken by a mix of 3G
  enhancements, WiMax and WiFi.
• E.g., the Japanese plan for 4G (OFDM, 3-6 GHz
  band, 100 Mbps) is almost identical to the
  specification for 802.11a
• 4G systems, if realized, can be economically
  deployed only in high-density areas.
• A further increase in air interface data rate is
  pointless without better backhaul technologies.
• E.g., insufficient speed of ADSL
• There may not be sufficient economic
  justification for the development of a completely
  new standard like 4G.
• Instead, might expect to see novel enhancements
  to the current standards making up the complete
  picture.
• E.g., WiFi-like cellular, cellular-like WiFi,
  femto-cell network

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Prognosis for Cellular

• A long period of stability, with profitable
  operation and deployment of 3G, is expected.
• The likelihood of dramatically new or
  destabilizing technologies appears to be low.
• There appear to be few threats to cellular
  revenues, with the exception of in-building voice
  calls transferring to W-LAN over time.

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Short-range Devices

• Potential applications for short-range devices
  are those that are not well suited to cellular.
• Networking around the office or home
• High-speed data transfer
• Cable replacement
• Machine-to-machine communications

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Prognosis for Short-range Devices

• W-LANs and BlueTooth will dominate the
  short-range devices market, providing building
  networks and device-to-device connectivity,
  respectively.
• WiMedia/UWB is still a developing technology and
  it is unclear whether there are sufficient
  applications that need its very high data rates.
• Zigbee is likely to succeed as a niche standard
  for specific applications where widespread
  interoperability is not needed but battery life
  is critical.
• RFID is used in quite different applications from
  other short-range devices. There is little reason
  why it cannot continue to be successful.

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How People React to New Technologies
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How People React to New Technologies

• A new service or product might take 4 to 10 years
  to reach mass adoption. Adding 5 years for the
  standardization and development, it might take 15
  years from conception to large-scale success.
• It is unlikely that total communications spending
  will grow by more than 0.15 of household income
  per year.

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Spectral Efficiency is Approaching Limit

• Under some assumptions, Shannons law yields
• where number of users that can be
  supported
• total available bandwidth
• user bit rate
• closeness to the Shannon
  limit
• For a system with 1, 1MHz and
  10 Kbps, 142 calls.
• 3G systems with HSDPA enhancement attain 30-50
  calls per cell, delivering about a third
  of the maximum capacity achievable.
• Efficiency of wireless systems is approaching
  fundamental limits. Reaching further these limits
  through technology is not easy.
• A relatively easy way to drastically increase
  the capacity is to use smaller cells (micro,
  pico, femto etc.) and not better technology.
• While there is some prospect that MIMO might
  increase capacity beyond these, this prospect
  seems relatively small, especially its benefits
  decline in a small-cell environment.

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Key Technical Observations Empirical Laws

• Moores law
• Best industry prediction at present suggests that
  the growth trends will slow around 2010 and may
  stop altogether around 2016.
• Use of multiple parallel processors may allow
  some further improvement, but they are costly,
  power hungry and difficult to work with.
• Steady progress but no key breakthrough is
  expected in areas such as processing power, hard
  disk, batteries etc.

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Key Technical Observations Empirical Laws

• Edholms law
• Data rates for three communications categories
  (wired, wireless and nomadic) increase on similar
  exponential curves, the slower rates trailing the
  faster ones by a predictable time lag.
• Key is its prediction that wired and wireless
  will maintain a near-constant differential in
  data rate terms, although nomadic and wired seem
  to gradually converge at around 2030.
• The law predicts that, in 2010, 3G, Wi-Fi and
  office LAN will deliver around 1Mbps, 200 Mbps
  and 5 Gbps, respectively.

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Key Technical Observations Empirical Laws

• Coopers law
• The number of voice calls carried over radio
  spectrum has doubled every 30 months for the past
  107 years, implying that the effectiveness of
  spectrum utilization in personal communications
  has improved a million times, i.e., , since
  1950.
• A 15 times by allocating more spectrum, a 5 times
  by frequency division, a 5 times by enhancing
  modulation techniques
• The lions share of the improvement, a 2700
  times, was the result of effectively confining
  individual conversations to smaller and smaller
  areas by spatial division or spectrum reuse
• Despite being close to the Shannon limit, there
  is no end in ever increasing wireless capacity if
  we are prepared to invest in an appropriately
  dense infrastructure.

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Key Technical Observations Empirical Laws

• Metcalfes law
• The value of a network equals approximately
  (or ) where is the number of users of
  the system.
• Unlike Moores or Edholms law, Metcalfes does
  not have a time limit to it. It will likely apply
  to a wide range of new networks in the future as
  new types of devices and networks are invented.

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Technologies Lowering Cost Backhaul

• Cells have to be connected back into the
  infrastructure via backhaul.
• More costly as cells gets smaller.
• Backhaul technologies
• Cabling (copper, coaxial or fiber optic)
• Fixed wireless
• Wireless Mesh
• There are no significant technological changes
  expected that can lead to reduced backhaul costs
  and availability.
• The exception is potential advances of wireless
  mesh technology but it requires technological
  innovation to overcome its capacity and delay
  problems due to multi hopping and
  self-interference.

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Emerging Communications Techniques

• Disruption-tolerant network (DTN)
• Software-defined radio (SDR)
• Cognitive radio (CR)
• Opportunistic communications
• Relays
• Mesh/ad-hoc network
• Cross-layer control

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Disruption-tolerant Networks (DTN)

• Provide useable and useful communications across
  networks that are frequently disconnected and/or
  has no stable end-to-end path due to mobility,
  density, attack, disaster or environmental
  conditions.
• Use store and forward protocol and the concept of
  bundle.
• Can provide increases in both availability and
  capacity.
• Form an extremely important communication
  protocol, but it will be a number of years before
  operational deployment is practical.

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Disruption-tolerant Networks (DTN)

• Human mobility models
• End-to-end delay

Brownian motion
Levy walk
Random waypoint
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Software-defined Radio (SDR)

• Many future visions of wireless communications
  involve multi-modal devices connecting to a wide
  range of different networks or devices modifying
  their behavior as they discover new types of
  network.
• The current approach, incorporating the chipsets
  from each of the different standards into the
  device, works well but it is intrinsically
  inflexible.
• SDR is for communication devices to be designed
  like computers with general-purpose processing
  capabilities and different software for different
  communications.
• In the future this flexibility might enable the
  more efficient use of the spectrum through rapid
  deployment of the latest radio technologies.
• Issues with SDR implementation, particularly at
  terminal
• Difficulties in implementing broadband antennas
• Lack of sufficient processing power
• Insufficient battery power
• High cost

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Software-defined Radio (SDR)

• In practice, the benefits of SDR appear
  relatively minor compared to the issues.
• The current approach of multi-modal devices works
  well and will likely always be less expensive
  than a general-purpose SDR radio.
• Further, since new technologies are generally
  introduced much less frequently than users
  replace handsets, there is little need for a
  handset to download a new standard.
• Because of this, we do not expect true SDRs
  that can reprogram their radio at handsets during
  the next two decades.
• We do, however, expect handsets to be able to
  download a wide range of new applications.
• We also expect SDR base stations that can modify
  their behavior as hey discover new types of
  network or standard.

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Cognitive Radio (CR)

• Three approaches to spectrum scarcity
  amelioration
• Unlicensed bands e.g., ISM band
• Underlay must operate below the FCC Part 15
  noise limit and must use a very broadband carrier
  (at least 500 MHz), e.g, UWB
• Overlay dynamic usage of previously allocated
  spectrum when non-allocated users can prove that
  they will not disrupt the incumbent, e.g., CR
• CR has been defined by ITU as a radio or system
  that senses, and is aware of, its operational
  environment and can dynamically and autonomously
  adjust its radio operating parameters
  accordingly.
• The premise for CR is the observation that
• Effectively all the spectrum of interest has been
  allocated, thereby firmly establishing spectral
  scarcity
• Most of the spectrum, in most of the places, most
  of the time is underutilized
• CR is sometimes described as frequency-agile
  radio.

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Cognitive Radio (CR)

• Spectrum utilization for two of the USAs busiest
  cities
• The net spectrum utilization is 17.4 for Chicago
  and only 13.1 for New York.
• This suggests considerable opportunity for the
  deployment of overlay solutions based on CR.

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Cognitive Radio (CR)

• Will CR work? It may not work well.
• One of the key challenges is to overcome the
  hidden terminal problem.
• The problem can be solved by the base station
  transmitting beacon, indicating the spectrum
  band is free.
• Such an approach requires central management by
  the owner of the band including a choice as to
  whether they wish to allow secondary access and
  if so under what conditions.
• Is the spectrum needed? There is little need.
• 3G operators in 2005 were still typically only
  using 50 of their spectrum allocation.
• Additional 3G spectrum was promised at 2.5-2.7
  GHz and at UHF after analog TV switch-off.
• Cellular demand may eventually fall as more
  traffic flows to W-LANs.

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Opportunistic Communications

• Opportunistic scheduling

User 1
Fading channel
User M
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Opportunistic Communications

• Opportunistic routing
• Source broadcasts each packet without intended
  receiver.
• Learn the set of nodes which actually received
  the packet.
• A receiver in the set that is closest to the
  destination is selected to forward the packet.
• This continues until the destination receives the
  packet.
• Opportunistic routing provides more throughput
  than conventional routing
• Each transmission has more independent chances of
  being received and forwarded.
• Take advantage of transmissions that reach
  unexpectedly far.
• TX counts In opportunistic routing,
  (1-(1-0.25)4)-11 2.46
• In conventional routing, 41
  5

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Relays

• New generation of cellular requires dense BS
  deployment for the following reasons.
• Higher data rates can be attained by a smaller
  cell and a higher carrier frequency.
• Transmission at high carrier frequency (gt 2GHz)
  is vulnerable to non-line-of-sight environment
  such as metropolitan area.
• However, it is unacceptable due to its high
  deployment and maintenance cost.
• A cost-effective alternative is multi-hop
  relaying approach.
• Dense deployment of cheap relay stations (RS)
  with low transmit power
• Multi-hop wireless connection to BS, forming
  wireless mesh

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Relays

• Benefits of relays
• Low cost compared to BS deployment
• Coverage and fairness enhancement
• Unknowns and challenges
• If RS-RS and RS-BS transmissions use the same
  radio with MS-BS and MS-RS transmissions, total
  system throughput may decrease.
• RSs act as additional interference sources to
  neighboring cells so that ICI becomes more severe
  and total system throughput may decrease unless
  ICI is tightly managed.
• Cross-layer control of wireless mesh network is a
  big challenge.

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Wireless Mesh Networks (WMN)

• A wireless inter-network of various sub-networks
  including Wi-Fi networks, cellular networks,
  WiMax networks, sensor networks etc.
• A wireless backhaul network for Wi-Fi networks,
  cellular networks, WiMax networks etc.
• Many other application areas including community
  networking, enterprise networking, home
  networking etc.

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Wireless Mesh Networks (WMN)

• The current 802.11-based mesh technology cannot
  meet the promise.
• Insufficient capacity even with multiple channels
• Unfairness depending on path length
• No proven wireless multi-hop protocol stack
• TCP from wired Internet, routing protocols (AODV,
  OLSR, DSR etc.) from MANET and MAC from Wi-Fi
  network
• A clean slate protocol stack that can squeeze
  most performance out is necessary to meet the
  promise. Its design involves
• Understanding of optimal interaction between
  transport, routing, MAC (link scheduling, power
  control) and PHY layers
• Finding distributed algorithms and protocols that
  can most closely approximate the optimality
• Understanding of multi-link interference and
  finding maximal independent link sets in a
  distributed manner
• Understanding of optimal interaction between mesh
  links and access links if they share the same
  radio

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Cross-layer Control

• Wired multi-hop networks
• Network utility maximization
• Link capacity is given and constant
• Flow control problem at transport layer

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Cross-layer Control

• Lagrangian function
• Dual problem
• Dual decomposition
• Flow control at source
• Congestion price at link
• TCP is an approximation of this dual decomposition

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Cross-layer Control

• Wireless multi-hop networks
• Long-term network utility maximization
• Link capacity is time-varying and a function of
  resource control
• Joint rate, power allocation and link scheduling

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Cross-layer Control

• Lagrangian function
• Dual problem
• Dual decomposition
• Flow control at source
• Scheduling/power control at link
• Congestion price at link
• Joint MAC and transport problem
• Distributed scheduling/power control is a
  challenge